The invention relates to a method for bending radiation shielding glass and a method for thermally prestressing a radiation shielding glass or a glass body consisting thereof.
Within the scope of this application, a “radiation shielding glass” is understood as meaning a glass which meets the requirements for lead glass panes according to the German and European standards DIN EN 61331-1 (August 2006 version) and DIN EN 61331-2 (April 2002 version), i.e. the attenuation equivalent in mm Pb must not be less than 0.22 times the minimum permitted lead glass plane in millimeters. The above standards are based on the international standard IEC 61331-1:1994 and IEC 61331-2:1994 and are harmonized therewith.
Such radiation shielding glasses are used as shielding glasses against X-rays and gamma rays in the medical and industrial sector. They are installed in particular in radiation shielding doors and windows. The attenuation equivalent is determined according to the standard by comparison between the radiation shielding effect of a lead glass plate and a plate comprising radiation shielding glass. An attenuation equivalent of 0.22 means that the radiation shielding glass at its permitted minimum thickness still has 22% of the shielding effect of a lead glass plate of the same thickness.
In the narrow sense, a radiation shielding glass is understood as meaning a glass which, according to the above standards, has an attenuation equivalent of at least 25%, preferably of at least 28%, for a nominal thickness between 6.5 and 25 mm at a tube voltage of up to 200 kV with 1.2 mm Cu total filtering.
Said glass may be, for example, a glass which has a heavy metal oxide content of at least 50% by weight. It may be, for example, a heavy flint glass which preferably has a lead oxide content of at least 60% by weight.
As a result of this high heavy metal oxide content, such glasses have a high density (more than 4000 kg/m3 or even more than 4900 kg/m3) and a relatively low transformation temperature (<500° C.).
Such glasses moreover have reduced mechanical and chemical stability or strength of the surfaces compared with conventional soda-lime glasses.
A 2- or 3-dimensional deformation of such glasses and also thermal prestressing of such glasses are not possible by known bending methods (cf. for example U.S. Pat. No. 5,372,624).
This document discloses an apparatus and a method for bending a glass plate, the glass plate being held in a bending cell in an upper bending mold perpendicularly to an annular counter-mold which has an element for supporting the middle part of the glass plate in the bending cell. The annular counter-mold is raised in order to press the glass plate against the upper bending mold. The bending temperature here is about 650° C.
A radiation shielding glass cannot be bent in this way. Rather, particular measures have to be taken in order to take into account the particularly high weight and the low transformation temperature.